Because of low price and high reliability, a lead frame package has been used in the field of the ICs package for a long time. However, as ICs products are endlessly speeded and shrank, the conventional lead frame package has become unfashionable, at least for some efficiency-concerned ICs products. Therefore, the ball grid array (BGA) and chip scale package (CSP) have become a new choice for package. The BGA is widely applied to chips with large I/Os and chips that need better electronic properties and heat efficiency (for example, central processing unit and graphic chips). The CSP has also been wildly used in portable products, having main concerns for footprints, package profile and package weight.
However, for small I/Os the lead frame package still occupies the quite large market share, because it provides the solutions of the low cost efficiency. Because of having quite long inner leads and outer leads, the conventional lead frame package cannot provide the chip-scaled, small package volume solutions. Thus, the semiconductor package industry created a leadless package of which the footprint and package volumes are shrank tremendously.
FIG. 1 is the cross section of a known leadless package 10. Compared with the known gull-wing and J-leaded type packages, a plurality of leads 11a of the leadless package 10 is placed under the bottom of the leadless package. The chip carrier 11b of the leadless package 10 is exposed in the package bottom to provide better heat-radiating efficiency. The chip 12 is attached to the chip carrier 11b by using silver epoxy, and the chip 12 is electrically connected to a plurality of leads 11a. 
Eliminating the outer leads, the leadless package 10 has the features of low profile and low weight. Besides, because the lead length decrease results in the comparative decrease in resistance, inductance and capacitance, the leadless package 10 is very suitable for the high frequency package operated at several Giga Hertz to tens of Giga Hertz. Due to the current well-developed materials, the leadless package is a very price-competitive package technology. The above-mentioned properties make the leadless package very suitable for communication products (for example, mobile phone), portable products (for example, personal digital assistance, PDA), digital camera and information appliances (IA).
The package 10 is generally installed in a substrate, for example, a printed circuit board, by the surface mount technology (SMT). In details, the exposed leads 11a in the bottom of the package 10 are mounted to the corresponding pads 18 on the printed circuit board 16 by the solders 14. The current problem in the SMT operation of the known leadless package is that the exposed area of the lead 11a in the bottom of the package 10 is too small so that the soldering quality and reliability are significantly decreased. Although the fillet height can be increased through increasing the thickness of the lead 11a, the thickness of the lead 11a is confined (generally to about 0.15 mm) because the low profile is generally necessary for the leadless package. As shown in FIG. 1, the lead 11a on a side of the package may provide the height of only 0.15 mm for soldering. It leads to bad soldering strength.